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Where on his back is the hair loss? Lower back?
Is the skin in that area normal color or red?
The most common cause of hair loss, on the lower back, is flea allergy dermatitis. If cats are allergic to flea saliva, one flea bite is all it takes to make the hair come out. The cats often pull the hair out when trying to get at the fleas. I would apply a dose of "feline advantage drops." You will need to apply one dose for the next three months because the fleas have already laid eggs and they will be hatching over the next few months. You can buy advantage online, at a vet clinic, or at petsmart.
To reduce the inflammation you can spray hydrocortisone spray on the affected area three times a day.
You can give an antihistamine to help with itching. The easiest one to do is 1/4 of a 10 mg zyrtec tablet once day.
He may have rubbed his back on something and it scraped the hair off.
It could also be ringworm.
Ringworm is a type of fungus that causes circular patches of hair to fall out. Sometimes it can make large areas of hair fall out. You will need to go to your vet to get this one diagnosed. Your vet will pluck some hairs from an affected are and culture the hairs to see if there is any fungal growth.
I hope this helps.
Flea collars do not work at all. Sorry.
He could also have a bacterial, or yeast, skin infection.
Like I said, the most common cause of hair loss over the back is flea allergy dermatitis so I would try treating for that first.
If he is not improving in 2-3 days then he will need further testing at a vet clinic to check for a fungal or bacterial infection.
It sounds like he has "hot spots" which are localized areas of superficial skin infection.
The easiest thing to do is to take him to the vet where they will give him a cortisone injection, for inflammation, and antibiotics to fight infection.
If that is not an option, you can apply hydrocortisone and neosporin (antibiotic cream) to the affected areas three times a day. If he licks it off, it is not toxic but you will want to put a plastic cone collar/elizabethan collar on him so the medication stays on the skin. You can buy one of the elizabethan collars at the pet store.